This invention relates to a control circuit for an electric motor, which automatically turns off the motor during a motor underload condition.
Electric motors are frequently utilized to drive a water (or other liquid) pump. In some installations of this nature, damage to the pump may occur if the supply of water to the pump is disrupted for any reason. For example, a motor-pump for circulating water in a home spa or a jetted bathtub may have its water intake clogged, or the motor-pump may be turned on while there is no water in the tub or spa, with the result that the motor-pump runs with a serious underload. The pump frequently has a rotary seal which is lubricated by the water being pumped, but in the instance where the pump is running without water, the seals will run while dry. The result is that the seals become hot, and they may become permanently damaged.
The foregoing problem has been known in the prior art and is discussed, for example, in J. L. Tibbits et al. U.S. Pat No. 4,420,787. This patent proposes a protector wherein the phase angle between the applied voltage and the current are sensed to provide an indication of the power factor. It is known that the power factor changes with the load on the motor, and consequently a motor underload condition may be detected from the value of the power factor.
However, it is also known, and mentioned in the above patent, that the power factor is influenced by the value of the line voltage. The patent proposes to compensate for line voltage variations by measuring the value of the line voltage times the power factor, but this solution is considered to be inadequate because it does not provide compensation over a sufficiently wide range of power factors.
The Gephart et al. U.S. Pat. No. 4,123,792 proposes to measure amperes times power factor in order to protect a motor in a heat pump. This proposal is also considered inadequate because it requires an accurate measurement of motor current, and a different protector would be required for each motor model.
It is important that such a protection system which functions by sensing the power factor be able to compensate for line voltage variations. If there were no compensation of this nature, the system would be subject to false tripping caused by line voltage fluctuations.
It is a general object of the present invention to provide an improved protection system that senses the power factor and adequately compensates for changes in motor power factor due to line voltage variations.